The explosion seems to have been caused by a gas leak that was reported Wednesday morning from one of the two buildings, which contain a church, a piano shop and several residential units, officials and witnesses said.

"The walls started to crumble and the windows were gone," said Aisha Watts, 37, who was trapped in her apartment next to the blast site when rubble blocked the door. "I thought I was going to die. I thought about my three kids and said, 'I can't die.' I started yelling, 'Help me, help me!'"

A neighbor kicked her door down and freed her, she said.

Mayor Bill de Blasio said at a midday press briefing that "there are a number of missing," but wouldn't give a specific number saying it was difficult to track down people who might have gone to work or elsewhere. "God forbid there are some in that building," he said.

Hundreds of firefighters were still trying to extinguish the blaze as of 3 p.m., but search and rescue crews were already combing through the rubble searching for survivors. Sources said it could take "a few days" to complete the search.

"It's going to be a long extended operation and we want to make sure we can get through that debris as quickly as possible," said FDNY Commissioner Sal Cassano.

A hotline for those looking to find out about missing relatives was being set up, but people should call 311 (or 212-639-9675 outside of New York City) for information in the meantime, officials said.

The Red Cross set up a center for affected residents and their families Wednesday afternoon at P.S. 57, 176 E. 115th St.

At least two people were listed in critical condition and at least five people suffered serious injuries but were listed in stable condition as of 1:15 p.m., according to the FDNY. About a dozen people are missing.

The victims, including at least five children, were treated at Harlem, Metropolitan, Mt. Sinai and St. Luke's hospitals, the FDNY and hospitals said.

The explosion shattered the mid-morning calm and came with hardly any warning, witnesses said.

"This is a tragedy of the worst kind because there was no indication in time to save people. We've lost two people already," de Blasio said at the noon briefing.

A tenant at 1652 Park Ave. called Con Edison at 9:13 a.m., less than 20 minutes before the explosion, to report that she smelled gas and that it seemed to be coming from outside, utility officials said.

Two Con Edison crews were dispatched to the scene two minutes later, but found the two buildings leveled by the time they arrived, officials said.

The neighbor's call did not raise alarms at Con Edison, which will summon the FDNY if the leak appears serious enough, Con Edison CEO John McAvoy said.

"This did not meet that threshold," he said. "It did not require that upgraded response...based on the information we had."

The 1646 Park Ave. building contained residential units and a piano shop, Absolute Piano, at its base, records show. Spanish Christian Church was next door at 1644 Park Ave. and was also destroyed.

"It sounded like the world fell," said Isabel Castillo, 67, who was a block away at a Jehovah's Witness church. "We were very scared."

A man walking into one of the buildings got knocked to the ground by the force of the explosion, according to Anthony Ferguson, who was standing on a nearby corner.

"The first thing I heard was a boom. The next thing I saw was the smoke," Ferguson said. "Then I saw bricks raining down on the church."

A neighbor was in her home near the blast site at the time of the explosion.

"You heard a loud explosion and then my whole building shook. You could feel the force," said Theresa Gotton, a 27-year-old member of the National Guard.

Gotton ran outside and saw a massive pile of burning rubble, she said.

Emergency responders passed by her while carrying a person on stretcher, she said.

"We thought it was an earthquake because everything in the store shook," said Golam Bhuiyan, 23, who works at the UPS store at 116th Street and Lexington Avenue. "All the windows were shaking and looked like they were bending."

Bhuyian ran outside and saw glass falling from windows and people choking on clouds of black smoke.

Crews with the Department of Buildings, Con Edison and Office of Emergency Management all responded to the scene.

The National Transportation Safety Board, which investigates gas pipelines, held a press conference Wednesday evening, promising to find out what caused the investigation.

A Con Edison spokesman said that the cast iron pipe that feeds gas into the area dates back to 1877. They are also looking into whether a failure in their infrastructure caused the explosion.

The Department of Buildings said it did not have any open work permits on either of the leveled buildings, officials said.

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